Catalytic cathodic hardening of oxide films

ABSTRACT

OXIDIC FILMS ON STAINLESS STEEL AND SIMILAR ALLOYS FORMED BY TREATMENT OF THE ALLOYS IN AQUEOUS SOLUTIONS OF SULFURIC ACID AND PITTING INHIBITING OXIDIZING AGENTS ARE HARDENED BY BEING TREATED AS CATHODES IN AQUEOUS ELECTROLYTES CONTAINING HEXAVALENT CHROMIUM AND A SUBSTANCE CAPABLE OF PROMOTING THE DEPOSITION OF OXIDIC CHROMIUM DEPOSITS AT THE CATHODE IN PREFERENCE TO CHROMIUM METAL.

United States Patent Office Patented Aug. 27, 1974 3,832,292 CATALYTICCATHODIC HARDENING F OXIDE FILMS William Heald Sutton, Stourbridge,Thomas Ernest Evans, Solihull, and Anthony Christopher Hart, Sedgley,England, assignors to The International Nickel Company, Inc., New York,N.Y.

No Drawing. Filed Oct. 30, 1972, Ser. No. 301,810 Claims priority,application Great Britain, Oct. 12, 1972, 47,048/72 Int. Cl. C23b 11/00US. Cl. 204--56 R 12 Claims ABSTRACT OF THE DISCLOSURE Oxidic films onstainless steel and similar alloys formed by treatment of the alloys inaqueous solutions of sulfuric acid and pitting inhibiting oxidizingagents are hardened by being treated as cathodes in aqueous electrolytescontaining hexavalent chromium and a substance capable of promoting thedeposition of oxidic chromium deposits at the cathode in preference tochromium metal.

The present invention is concerned with hardening oxidic films formed oniron-base, chromium-containing, corrosion-resistant alloys, e.g.,stainless steels, by treatment of said alloys in aqueous sulfuric acidsolutions containing a pitting inhibiting oxidizing agent(advantageously chromic acid).

A known method of treating stainless steel and other chromium-containingalloys comprises two main steps, in the first of which an oxidic film isformed on the surface of the alloy by immersion of the alloy in anaqueous solution of chromic and sulfuric acids, with or without otherconstituents, and in the second of which the alloy bearing the film issubjected to electrolysis as the cathode in an electrolyte from whichchromium can be deposited. This method is the subject of US. applicationSer. No. 114,357, now US. Pat. 3,755,117 and 252,459, now US. Pat.3,766,023, filed in the name of Anthony C. Hart, in which it isexplained that the electrolytic treatment is short, lasting for a periodof time adequate to harden the film, but not so long that any chromiumbecomes visible on the surface as a white deposit.

In practice of the Hart process, there is an increasing tendency forchromium metal to be deposited from. aqueous electrolytes containingchromic and sulfuric acids as the current density increases, and, at thecomparatively low current density required to ensure that no chromiummetal is deposited, it is difficult to harden the film completely. Ourobject in the present invention is to eliminate or at least to reducethe deposition of metallic chromium on cathodes during electrolysis ofsolutions containing hexavalent chromium.

It is an object of the invention to provide a novel process forhardening oxidic films on stainless steel and similar alloys.

Generally speaking, the present invention contemplates a process forhardening an oxidic film produced on an iron-base, chromium-containing,corrosion-resistant alloy by treatment in an aqueous sulfuric acid bathcontaining a pitting inhibiting oxidizing agent comprising treating as acathode the surface of such an alloy bearing such an oxidic film in anaqueous solution containing hexavalent chromium (introducedadvantageously as CrO and an agent capable of promoting the cathodicdeposition of chromium oxide in preference to metallic chromium. Inhighly acidic aqueous chromic acid electrolytes, e.g., baths containingabout 25 to about 750 grams per liter (gpL) of CrO ions produced byaddition of substances from theigroup of CrCl HF, HNO H PO and CH COOHand ions derived from reaction of these substances with water and/orhexavalent chromium are capable of promoting the cathodic deposition ofoxidic deposits such as chromic oxide, hydrated chromic oxide or chromichydrpxide in preference to chromium metal.

When employed in this specification and claims, the term iron-base,corrosion-resistant, chromium-containing alloy includes stainless steelsand other iron-containing alloys which also contain greater than about11% and up to about 30% by weight of chromium. Stainless steels can beferritic or austenitic and usually contain about 13% to about 25% (byweight) chromium.

It is thought that when the metal carrying the film to be hardened ismade the cathode in a hardening solution as contemplated in the presentinvention and cathodic electrolysis carried out, the hexavalent chromiumsolution is cathodically reduced and an oxidic deposit such as oxide,hydrated oxide or hydroxide is formed in the pores of the film. Local pHchanges brought about electrolytically in proximity to the film areconsidered to assist in the oxide deposition. The color of the oxidedeposited has little or no effect on the color of the film, although thecolor of the film is usually slightly advanced in shade.

Examples of mirror finish Type 304 stainless steel plate given a bluefilm coating by immersion in a chromic and sulfuric acid solution, andhardened by cathodic electrolysis in an electrolyte according to theinvention will now be described. in these examples, the experimentalvariables, apart from the solution, include current density, time ofprocess and temperature, and the effects of the hardening treatment areto harden the film and slightly to advance the color of the film inshade. Undesirable hardening effects are chromium metal deposition,oxide deposition on top of the film and too great or uneven coloradvancement. Hardness of the film is measured in the examples by astandard rub-test which consists of rubbing the film surface with apencil-type eraser, preferably Remington, loaded with a 400 gram weight.As a basis for comparison, an unhardened film would fail in one ,or tworubs, whereas a film hardened according to the, invention can withstandup to as many as six hundred rubs before failure.

- EXAMPLE 1 Chro'mic acid plus chromic chloride in aqueous solution wasemployed as the hardening electrolyte. The solutiomwas made up of 250gpl. of CrO and 2.1 gpl. of CrCl '6I-I O to provide a 2.5M CrO and0.025N chloride ion, i.e., a molar ratio of about 100 to 1 hexavalentchromium to chloride ion.

It is thought that all chromic acid based hardening solutions work onthe same principle as the Hart Gro /H solutions. The important cathodereactions are: 1) reduction of Cr Cr (2) PH discharge leading to pH riseand the precipitation of Cr as oxide or hydroxide; and (3) a proportionof the Cr is reduced to Cr and deposited as chromium metal. Whereas thepresence of SO anions seems to catalyze both the Cr Cr and Cr Cr, it canbe that other ions can preferentially or solely catalyze Cr Cr Testconditions are as set forth in Table I.

1 Amperes per square decimeter.

The data in Table I show it is possible to obtain remarkably increasedhardness using CrO /Cl of 100:1.-.

The increased hardness does not result in unacceptable coloradvancement. Provided that the process is not operated for too long atime, no deposit is formed on top of the color 'fi1m. Raising theoperating temperature to 40 C. or 60 C. is beneficial, enabling longerprocess times to be used without surface deposition.

Changing the CrO /Cl ratio to 20:1 ofiers no advantage.

EXAMPLE 2 Chromic acid plus hydrofluoric acid in aqueous solution wasemployed as the hardening electrolyte. The electrolyte was made up with250 gpl. of CrO and 2.5 gpl. of 40% hydrogen fluoride in water toprovide a 50 to 1 molar ratio of CrO to fluoride ion. Test results areset forth in Table II.

TABLE II Hardness Current (No. of nsity Time Temp. rubs to (an/din!) 1(min.) 0.) failure) Remarks 2 2 20 200. Trace of Cr deposit aroundedges; slight color advancement. 2 20 150 Slight color advancement. .1so 20 300 Do. 0.05 30 20 360 Do.

At 20 C. it is advantageous to work at lower current densities to avoidCr metal deposition. Surprisingly the lower current densities producebetter hardening than higher current densities. As in the CrOg/Clsolution, better hardening is obtained than in CrO /H SO solution.

Some experiments were carried out using a 20: 1 ratio of CrO :F, withsimilar results.

EXAMPLE 3 Chromic acid plus nitric acid in aqueous solution was employedas the hardening electrolyte. The test solution contained 250 gpl. CrOand 2.2 gpl. of 70% by weight HNO solution giving a 100:1 molar ratio ofCrO :NO Table III shows the results obtained.

The data in Table III shows that the hardness obtained in chromicacid-nitric acid aqueous electrolytes is comparable with that given bythe Gro /H 80 solution.

EXAMPLE 4 Chromic acid plus orthophosphoric acid in aqueous solution wasemployed as the hardening electrolyte.

4 (1) Solutions of CrO /H PO of the following compositions wereinvestigated:

CrO 250 gpl.: H PO 0.5 gpl., i.e., 500:1 ratio CrO 250 gpl.: H PO 2.5gpl., i.e., :1 ratio CrO 250 gpl.: H PO 5.0 gpl., i.e., 50:1 ratio CrO250 gpl.: H PO 12.5 gpl., i.e., 20:1 ratio (2) Each solution compositionwas tested at temperatures of 20 C., 40 C., and 60 C.

(3) Each solution and temperature was tested at current densities of0.1, 0.2, 0.5 and 1.0 a./dm. for 20 minutes. Typical results are setforth in Table IV.

TABLE IV Hardness Current (No. of ClO3/H3PO4 solution Temp. density Timerubs to ratio C.) (aJdJI'L (in min.) failure) The data in Table IV showthat the hardening was greater than that given by a CrO /H SO solution.The color advancement of the hardened films was acceptable and no oxideor chromium metal was deposited on top of the color film.

The following trends within the ranges of variables tested wereobserved:

EXAMPLE 5 Chromic acid plus acetic acid in aqueous solution was employedas the hardening electrolyte. A solution of 250 gpl. CrO plus 5.25 gpl.acetic acid, i.e. a molar ratio of 28.5: 1, was used at 40 C. fortreatment times of from 2 minutes at a current density of 15 a./dm. to15 minutes at a current density of 4 a./dm.

H able V shows some of the results obtained.

TABLE V Hardness (No. of Current Time rubs to density (a./dm. (in min.)failure) 1 Solution allowed to stand for 2 days.

The resulting hardnesses were comparable to or in excess of thoseachieved with chromic acid plus sulfuric acid hardening solutions, forexample, equal to or better than 100-150 rubs to failure of the film.There was some evidence that a freshly prepared solution was moreeffective than a solution that had been allowed to stand, possibly,because on standing acetic acid may have been oxidized by the chromicacid.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be under stood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to be withthe purview and scope of the invention and appended claims.

We claim:

1. A process for hardening an oxidic film on iron-base, chromium-cotaining, corrosion-resistant alloy, aid xidic film having been formed byimmersion of said alloy in an aqueous solution of sulfuric acid and apitting-inhibiting oxidizing agent comprising making said alloy bearingsaid film a cathode in an aqueous solution containing hexavalentchromium and an agent capable of promoting the cathodic deposition ofchromic oxide, hydrated chromic oxide or chromic hydroxide in preferenceto metallic chromium and passing sufiicient electric current across thesolution cathode interface so as to deposit said chromic oxide, hydratedchromic oxide or chromic hydroxide in said oxidic film to harden saidoxidic film.

2. A process as in claim 1 wherein the agent capable of promoting thecathodic deposition of chromic oxide, chromic hydroxide or hydratedchromic oxide in preference to metallic chromium is selected from thegroup of chloride ion, fluoride ion, nitrate ion, phosphate ion andacetate ion and ionic species derived from reaction of said ions withwater and hexavalent chromium.

3. A process as in claim 2 wherein the mole ratio of hexavalent chromiumto ion of said agent is about 20 to 1 to about 500 to 1.

\4. A process as in claim 1 wherein the agent added to the aqueoussolution is a chloride ion donor.

.5. A process as in claim 4 wherein the cathode current density is about2 to about 4 amperes per square decimeter and the molar ratio of CrO tochloride ion is in the range of about 100 to 1 to about 20 to 1.

6. A process as in claim 1 wherein the agent added to the aqueoussolution is a phosphate ion donor.

7. A process as in claim 6 wherein the cathode current density is about0.1 to about 1.0 ampere per square decimeter.

8. A process as in claim 6 wherein the aqueous solution is maintained ata temperature of about 20 C. to about 60 C.

9. A process as in claim 1 wherein the aqueous solution containinghexavalent chromium is a solution of chromic acid containing about 25 toabout 750 grams per liter of CI'O3.

10. A process as in claim 1 wherein the agent added to the aqueoussolution is a fluoride ion donor.

11. A process as in claim 1 wherein the agent added to the aqueoussolution is a nitrate ion donor.

12. A process as in claim 1 wherein the agent added to the aqueoussolution is an acetate ion donor.

References Cited UNITED STATES PATENTS 3,535,213 10/1970 Okada 2044lJOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl.X.R.

